The art of surveying involves the determination of unknown positions, or setting out of known coordinates using angle and distance measurements taken from one or more positions. In order to make these measurements, a surveying device or instrument frequently used is a distance measuring instrument with an integrated distance and angular measurement of the type which is called a total station, i.e. with combined electronic, optical and computer techniques. A total station is furthermore provided with a computer or control unit with writable information for measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail.
When performing distance measuring or surveying tasks using a distance measuring total station at a work site, naval work site, a construction work site or a mining work site, it is often desirable to lock and track a certain target, for example, mounted on a ground preparing machine. Thereby, it is possible to perform tasks such as tracking the ground preparing machine or measuring the distance to the machine. The target or reflector unit may, for example, be mounted on the roof of the vehicle.
In the prior art, there are a number of different reflector or target constructions, for example, a corner cube prism. A corner cube prism reflects back an incident beam in the opposite direction even if the angle of incidence is relatively oblique. Furthermore, in U.S. Pat. No. 6,185,055 a 360-degree reflector is shown. The reflector includes triple prisms arranged such that they cover an angular area of 360° in a plane.
However, it is often desirable to ensure that the measuring instrument identifies and locks onto the correct target. This is of particular interest when operating at a work place with multiple targets or in highly reflective, difficult environments. Therefore, there is a need within the art for a target for use in distance measuring and surveying applications that is capable of receiving a light beam emitted from a measuring instrument and for reflecting it toward the measuring instrument in a reliable way at the same time as the measuring instrument is able to secure that reflected light was received from the correct target.